What Causes Measurement Errors; Directivity - HP 8753E User Manual

What Causes Measurement Errors?

Network analysis measurement errors can be separated into systematic, random, and drift
errors.
Correctable systematic errors are the repeatable errors that the system can measure. These are
errors due to mismatch and leakage in the test setup, isolation between the reference and test
signal paths, and system frequency response.
The system cannot measure and correct for the non-repeatable random and drift errors. These
errors affect both reflection and transmission measurements. Random errors are measurement
variations due to noise and connector repeatability. Drift errors include frequency drift,
temperature drift, and other physical changes in the test setup between calibration and
measurement.
The resulting measurement is the vector sum of the test device response plus all error terms
The precise effect of each error term depends upon its magnitude and phase relationship to the
actual test device response.
In most high frequency measurements the systematic errors are the most significant source of
measurement uncertainty. Since each of these errors can be characterized, their effects can be
effectively removed to obtain a corrected value for the test device response. For the purpose of
vector accuracy enhancement, these uncertainties are quantified as directivity, source match,
load match, isolation (crosstalk), and frequency response (tracking). Each of these systematic
errors is described below.
Random and drift errors cannot be precisely quantified, so they must be treated as producing a
cumulative uncertainty in the measured data.

Directivity

Normally a device that can separate the reverse from the forward traveling waves (a
directional bridge or coupler) is used to detect the signal reflected from the test device. Ideally
the coupler would completely separate the incident and reflected signals, and only the reflected
signal would appear at the coupled output, as illustrated in the Ggure below, left "Ideal
Coupler".
However, an actual coupler is not perfect, as illustrated in Figure 6-2913. A small amount of
the incident signal appears at the coupled output due to leakage as well as reflection from the
termination in the coupled arm. Also, reflections from the coupler output connector appear at
the coupled output, adding uncertainty to the signal reflected from the device. The figure of
merit for how well a coupler separates forward and reverse waves is directivity. The greater
the directivity of the device, the better the signal separation. System directivity is the vector
sum of all leakage signals appearing at the analyzer receiver input. The error contributed by
6-56 Application and Operation Concepts
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Figure 6-29. Directivity
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o u t p u t
( b ) A c t u a l C o u p l e r